This invention relates generally to a method of sintering and densification of silicon nitride powders, and more particularly to sintering and densification of such powders in the absence of applied pressure.
In the past, the principal methods of fabricating silicon nitrade by powder techniques have been reaction sintering of silicon powder in a nitrogen atmosphere and hot pressing of silicon nitride powders containing densification aids.
In the reaction sintering process silicon powders are shaped to the desired configuration and thereafter heated in the presence of nitrogen to form a silicon nitride shaped structure. (See for example N. L. Parr, R. Sands, P. L. Pratt, E. R. W. May, C. R. Shakespeare and D. S. Thompson, "Structural Aspects of Silicon Nitride" Powder Met [8] 152-63 (1961). This results in closely maintained dimensional tolerances of the articles, but a comparatively low strength (less than 30,000 psi flexural strength) product.
When silicon nitride powder has been used as a starting material, the prior art techniques have required both heat and applied pressure to form the final end product. (See for example United Kingdom Patent No. 1,092,637.) High strength (approximately 100,000 psi flexural strength) articles are obtained from this method but complex shapes are expensive and difficult to obtain, and dimensional tolerances can be obtained only by machining.
There have been many previous attempts to sinter silicon nitride powder into dense shapes by heating alone in the absence of applied pressure in an effort to obtain both high strength and inexpensive more easily produced complex shapes. (See for example D. J. Godfrey, "The use of Ceramics in High Temperature Engineering" Metals and Materials Oct. 1968, p.305) However, these attempts have proved to be unsuccessful in the past. Basically, when utilizing conventional sintering techniques with the silicon nitride powder, it was observed that the silicon nitride powder, even with densification aids added thereto, would decompose, and not densify to any appreciable extent. It appeared that at sintering/densification temperatures the silicon nitride apparently decomposed rapidly to a gaseous product preventing densification to any appreciable extent. (See for example H. D. Batha et al. "Kinetics and Mechanism of the Thermal Decomposition of Si.sub.3 N.sub.4 " Journal of The American Ceramic Society, Vol. 56, No. 7 Pages 365-369)